Public health authorities are urging people to get vaccinated. The goal is to get 80 percent of all Americans and 90 percent of at-risk populations (the elderly and children, in particular) vaccinated every year. That would ensure “herd immunity,” meaning it would provide enough protection to stop the spread of the virus. Yet less than 60 percent of children and only 43 percent of adults were vaccinated last flu season. The result is lots of unnecessary illness.

This raises the question, why don’t we mandate the flu vaccination, if not for all Americans, then at least for everyone under 18? After all, every state mandates that children get immunized for measles, rubella, diphtheria, pertussis, tetanus and polio before enrolling in kindergarten.

One reason is while measles and other infections are acknowledged to be deadly, the flu is viewed as, well, just the flu. This perception is simply wrong. It may be rooted in many people’s inclination to call any viral infection or malaise they feel during winter “the flu.” But when you get the real influenza virus, you know it. You can be seriously debilitated by high fevers, sweats, shaking chills, deep muscle aches, very low energy and occasionally vomiting and diarrhea. The flu can be fatal especially for young children, the elderly and people with conditions like cancer or recent organ transplants that weaken their immune systems.

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The best way to minimize the spread of flu is to immunize children, adolescents and young adults.CreditDavid Goldman/Associated Press

Of course, it makes sense to mandate vaccination for measles; the disease can be deadly and its vaccine is lifesaving. In the 1950s, before vaccination became available, there were at least 319,000 infections and 345 deaths from measles each year. More recently there was a measles outbreak at Disneyland in 2014. The 667 cases of measles in the United States that year occurred largely because of anti-vaxxers. Fortunately, no one died. Indeed, in the past 15 years there has been only one confirmed death from measles in the entire country. The Disneyland episode propelled states to change their laws to make it much harder for parents to refuse vaccinations for their children.

So why do we mandate the measles vaccine but not the flu vaccine? Perhaps because the flu vaccine must be given every year and is sometimes, like this year, not that effective. However, this is all the more reason to impose a mandate.

The relationship between the effectiveness of the vaccine, the number of people who need to be treated and herd immunity is complex — and perhaps counterintuitive. The less effective a vaccine is, the more people you need to treat to produce herd immunity. For instance, if the vaccine is 80 percent effective, then vaccinating just 25 percent of the population provides effective herd immunity. But if the vaccine is only 30 percent effective — just short of this year’s level — around 70 percent of the population must be vaccinated.

Of course, some Americans hate government mandates, even when they are demonstrably beneficial. Rand Paul — a physician, no less — argued that mandatory vaccination is a violation of individual rights, claiming, “the state doesn’t own your children.” He called it “an issue of freedom.”

But the freedom to allow your child to contract and transmit a deadly disease is hardly a real freedom worth protecting. In 1905 the Supreme Court recognized the authority of states to enforce compulsory vaccination — in that case for smallpox. The court made it clear that the state may be justified in restricting individual liberty when “great dangers” threaten “the safety of the general public.” As the public outrage after the Disneyland outbreak showed, Americans do believe that getting a vaccine is a small imposition when a child’s life is at risk.

Today at least two states mandate flu immunization for children between 6 months and 5 years of age. In Connecticut, flu-related hospitalization rates for that age group declined 12 percent after the mandate was imposed. Indeed, evaluations show that the most effective way to minimize the burden of flu is to go one step further and “focus immunization programs on children, adolescents and young adults.” Vaccines are more effective in young people than in the old, so concentrating on them is the fastest way to engender herd immunity.

This would also save money, even after factoring in the cost of the vaccine. Researchers calculate that vaccinating children produces cost savings of more than $12,000 per child.

Starting next year, we should provide the vaccine free in all schools, preschools and day care centers — and require that all children under 18 get it before Thanksgiving.

What are we waiting for? Must more children die unnecessarily?

Ezekiel J. Emanuel is the author of “Prescription for the Future,” a partner at the Oak HC/FT health care investment company and a vice provost at the University of Pennsylvania, where Justin Bernstein is completing his doctorate.